1. Field of the Disclosure
The present disclosure relates to a composition for a high strength loose tube type fiber optic cable.
2. Description of the Related Art
A fiber optic cable is a cable having several optical fibers or optical fiber bundles surrounded by a sheath and suitably arranged to meet optical, mechanical and environmental conditions. It is a cutting-edge facility capable of rapidly transmitting a large amount of information over long distances. A fiber optic cable may be classified into various kinds depending on its classification criterion. In detail, the fiber optic cable may be classified into ribbon type fiber optic cables, tight butter type fiber optic cables, loose tube type fiber optic cables or the like according to the structure of optical fiber units and may also be classified into fiber optic cables for ducts, fiber optic cables for direct buried, fiber optic cables for aerial or the like according to the installation pattern.
Among them, the loose tube type fiber optic cable is generally configured as shown in FIG. 1. The loose tube type fiber optic cable is manufactured to include a central strength element 10 for endowing tensile resistance to the fiber optic cable, a plurality of fiber optic loose tube units 13 arranged to contact the central strength element 10, a reinforcing strength elements 15 surrounding the fiber optic loose tube units 13 and a sheath 16 surrounding the reinforcing strength elements. The central strength element 10 extends at the center of the fiber optic cable in the length direction of the fiber optic cable, and the fiber optic loose tube unit 13 is disposed to circumscribe the periphery of the central strength element 10 at regular intervals. In addition, the fiber optic loose tube unit 13 is composed of a plurality of optical fibers 11 and a loose tube 12 surrounding the optical fibers 11, and a jelly 14 is filled in a space formed in the fiber optic loose tube unit 13 for watertight configuration.
However, for environment-friendly and work convenience, the demand for gel-free all-dry type fiber optic cable is increasing. This fiber optic cable is configured as shown in FIG. 2. Talc powder and a water blocking yarn 24 are used in the fiber optic loose tube unit 23 of the fiber optic cable instead of the conventional gel for making it watertight.
Meanwhile, in order to protect the optical fiber 11 against external impacts and external environment (twist, compression, tension, temperature change or the like), the material used for the loose tube 12 must have excellent tensile resistance, excellent impact resistance, excellent flexibility, and small shrinkage or expansion. In order to satisfy these properties, conventional loose tubes were made of engineering plastic such as polybutylene telephthalate (PBT), polycarbonate (PC), polyamide (e.g., nylon-12) or the like. However, the engineering plastic is difficult to process and handle and is high in price. In addition, the engineering plastic has poor flexibility and is weak against water since it is hydrolyzed.
In order to overcome such drawbacks, a loose tube 12 having excellent flexibility and impact resistance has been manufactured by using a polypropylene-polyethylene copolymer. However, the polypropylene-polyethylene copolymer has bad workability since it has a low cooling speed and a high linear expansion coefficient in comparison to existing polybutylene telephthalate (PBT) or the like. In addition, the polypropylene-polyethylene copolymer deteriorates transmission characteristics of the optical fiber since its shrinkage rate after extrusion is great. In order to solve these problems, a technique of enhancing a cooling speed and minimizing a shrinkage rate by using a nucleating agent together with a polypropylene-polyethylene copolymer having a melt flow index (MFI) greater than 3 g/10 minutes and no greater than 10 g/10 minutes.
Meanwhile, with the development of a FTTH (Fiber To The Home) service, the demand on closures (or, pedestals) which play a role of protecting a contact point of a fiber optic cable against water or various environmental elements when the fiber optic cable is installed is increasing. In the closure, a fiber optic loose tube unit 13, 23 is wound circularly and stored for making an additional connection when necessary. Since the fiber optic loose tube unit is in a state where the central strength element 10, 20 and the sheath 16, 26 are removed, the fiber optic loose tube unit may be easily shrunken or expanded due to external environments (temperature, humidity or the like), and accordingly the length of the loose tube may decrease smaller than the optical fiber (EFL (Excess Fiber Length)>0) or increase greater than the optical fiber (EFL<0), which deteriorates a signal transmission capability of the optical fiber. In fact, many cases of inferior fiber optic cables using polypropylene fiber optic loose tube units have been reported, and it has been revealed that such inferiorities are caused since the loose tube of a fiber optic loose tube unit stored in the closure is shrunken to deteriorate the transmission capability of the fiber. As described above, in a fiber optic cable, the shrinkage rate of the loose tube is very important, and after inferior cases have occurred, Mid-span access-related standards have been established and managed in the U.S.A.